A conventional process called the Hitchiner counter gravity casting process provides a means to reduce gas defects in casts by sealing an investment tree within a vacuum chamber with a suction tube protruding from within the chamber. A metal suction tube is placed into molten metal and metal is pressed up into the mold void by atmospheric pressure. However, this conventional process required that ceramic molds be designed to withstand the pressure of the injected metal, otherwise ceramic mold shell failure would result. During a ceramic mold failure, a large transfer of liquid metal into the chamber (the chamber is capable of pressure and vacuum) would be difficult to avoid. Also, this conventional process is limited to pressures approaching one atmosphere of pressure gradient. In addition, features smaller than 0.5 mm present a challenge.
Another conventional Hitchiner casting process called Pneucast employs a chamber capable of high pressure (e.g., up to about 2500 PSI) and a mold positioned at the bottom of the chamber. After metal is introduced, high pressure is applied and the resulting castings have reduced porosity and higher strength. However, the chamber setup is not simple and a chamber may be lost for each casting. Also, the ceramic mold may not have a uniform distribution of pressure, and regions of tension result in the ceramic mold cracking. If the ceramic mold cracks, metal can also escape the mold cavity creating flash and potentially bonding to and/or damaging the chamber. In addition, the vacuum applied to the ceramic mold may not be of sufficient quality as molten metal is poured into the chamber.
Still another conventional method for making metal matrix composites uses a similar process to the high pressure Hitchiner process. Similar problems to the Hitchiner process are likely. Yet another method of applying pressure to a casting is centrifugal casting, which is conventionally used for jewelry. The centrifugal casting method results in the violent introduction of metal into the mold. Also, the ceramic mold is under tension during casting. In addition, thick-walled molds can lead to problems in cooling and applying a vacuum can present problems.
Most conventional metal casting processes are performed under conditions resulting in tension within the mold material. Well known to foundries, tension in ceramic or sand molds is not ideal, and must be minimized to ensure mold survival just long enough for the metal void to be captured.